This paper analyzes the operation of PowerMOSFETs in the 42-V-PowerNet and shows that very stressful conditions are encountered, which can lead to severe reliability problems. To enable thorough investigations by circuit simulations an accurate physics-based compact model of the devices is proposed: it includes all important electrothermal effects relevant to the description of the observed failure mechanisms. By means of an advanced thermal-modeling approach, multichip assemblies can be accurately described, including mutual heating effects between neighboring devices. Some properly chosen examples demonstrate the validity of the model and its usefulness for reliability investigations